I don't know whether Cougar is considered a high quality brand or not, or if the STE series is particularly good or bad.
That said, note on the specs page, it only provides a max of 34A on the 12V rail, or 408W max. So, consider it a 408W PSU. Barely within the recommendation.
That said, the recommendations are usually a little overkill. Assuming there's nothing crazy about the rest of the system (massive overclocked processors, several hard drives, etc), well, the GTX 960 is a 120W video card, so 400W is well more than enough for it and the rest of the system.